Adding visual effect to units in region

[WeaselChiseal]

Hello, lately i had some problems with a feature which would seem very simple. The idea is to add a visual effect at units overhead when it enters the region and to remove it when it leaves (only Heroes). Right now i'm adding and removing an ability based on human-unit spell "sphere" from triggering unit. Problem is that effect disappears and reappears with each unit attack (thats simply how "sphere" works) and it simply looks bad and sounds bad. I don't want to use aura ability since it appears in units interface. I also don't want to create a dummy unit with aura at the centre of region since its a more of a square than a circle. I can't tell how many units will be in there and for how long. Do any of you have an idea how to resolve this?